Earthworms are frequently referred to as a farmer’s friend because of the amazing benefits of earthworms in soil. The role of earthworms in soil includes tilling and aerating the soil. Their castings (i.e. worm poop) are full of beneficial microorganisms and can be used immediately by plants. Earthworms play an important role in the soil food web and are an important part of ensuring we have healthy soil. But, did you know that all earthworms aren’t the same? In this article, we are going to talk about the different types of earthworms and the roles of earthworms in the garden.
Table of Contents
Types of earthworms
Earthworms are classified into 3 ecological groups (epigenic, anecic and endogeic). What group an earthworm belongs to depends on where they are found in the soil, their feeding behaviour, and how they burrow through the soil.
- Epigeic earthworms are 1 to 2.5 mm in diameter, live and feed above the soil surface, forest litter. The earthworms in this group rarely burrow. When they do their burrows are not permanent and have very little effect on soil structure.
- Anecic earthworms are 4 to 8 mm in diameter, feed at the ground surface and decaying organic residues. These worms live in semi-permanent vertical burrows that are open to the soil surface.
- Endogeic earthworms are 2 to 4.5 mm in diameter These worms can usually be found 10 -15 cm below the soil’s surface. They ingest soil, dig extensive systems of horizontal burros to search foods and they immediately refill with their casts.
Composting worms
All composting worms that are bread in worm farms for the production of castings are in the epigenic category. They feed primarily on organic matter such as animal and plant waste and can often be found on or near the soil surface. While composting worms are great at breaking down organic matter, they don’t make permanent burrows in the soil. Thus, they aren’t good at aerating your garden soil.
How earthworms improve soil structure
Soil structure is defined by the way individual particles of sand, silt, and clay are assembled. Single particles when assembled appear as larger particles. These are called aggregates. Aggregation of soil particles can occur in different patterns resulting in different soil structures.
Soil structure affects the soil’s physical properties such as pore size and aeration. It also affects how well plants are able to grow by influencing how easily plants can uptake nutrients and root development. Additionally, it affects how water behaves in soil. Hydraulic conductivity, infiltration, water holding capacity of soils and erosion are all heavily dependent on the soil’s structure.
The biology of living soil, including earthworms, also affects soil structure. Earthworms consume and excrete plant and soil residues, and incorporate these into aggregates. This is how earthworms contribute to the formation and stabilization of soil aggregates. As a result, the physical conditions of the soil are improved to support plant growth.
Whether your soil is too loose or too compact, earthworms can help bring soil to a more ideal structure. One that is more stable and better able to support root growth. When earthworms move through the soil they change the soil’s structure by creating burrows and casts, and by breaking down organic matter.
How earthworms aerate the soil
Earthworm activity reduces soil bulk density by creating larger pores (with their burrows). These large pores permit water and gasses to flow into the soil. Earthworm burrowing can increase soil aeration and drainage.
How earthworms improve soil water holding capacity
Endogenic earthworms, in particular, do consume some soil. The earthworm’s soil gut passage can improve soil crumb structure and lead to enhanced water holding capacity. Additionally, the worm castings of composting worms are very good at retaining water in the soil. We’ve noticed in our garden our castings can hold more than their weight in water.
Benefits of worm castings
Earthworms translocate surface organic matter to deeper in soils and produce organo-mineral casts. This movement is particularly true for the Anecic earthworms that burrow vertically through the soil. Worms in this group tend to take food from the soil surface and carry it down into their burrows to consume it.
When worms produce castings in the garden it takes the organic material and incorporates it with the minerals. This results in the soil resulting in increased nutrient availability. Observations at Rothamsted Experimental Station, Hertfordshire, in the 1970s showed that increased numbers of earthworms in soil resulted in improved crop yields and a better quality of grassland.
Shallow working species (i.e. the epigenic group) are thought to be responsible for increased root growth near the surface of soils. Because of this, sprinkling a handful of castings early growth phase can be beneficial.
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Earthworm diversity in soils
In the garden, we need a wide variety of earthworms in healthy soil, not just composting worms. We need worms that can break down organic matter and make nutrients available to plants. We also need earthworms that create vertical and horizontal burrows to allow the exchange of water and air between the soil and the air above it.
Without earthworms, soil can become a thick mat of dead plant matter. When earthworms were well established, the surface mat will disappear. This results in greater heat exchange between the soil and air. It also reduces the daily fluctuations in temperatures at the soil surface.
Final thoughts
Soil structure is one of the most important properties affecting crop production. Earthworms improve the physical structure of the soil. This improvement results in increased drainage and aeration, enhanced soil fertility, greater recycling of nutrients, reduced runoff and better conditions for plant root growth.
Whether your soil is too loose or too compact, earthworms can help. Earthworms have been shown to bring soil to an intermediate mechanical state that is far more favourable for structural stability and root growth.
Worms and their castings enhance soil aggregation, while burrowing regulates soil physical properties.
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